[Haskell] A puzzle and an annoying feature
daan at cs.uu.nl
Thu Nov 25 09:50:05 EST 2004
Keean Schupke wrote:
> No, closed classes are different, here we are talking about lazy overlap
> resolution, so
> if at _call_ time only one instance fits we choose it. Closing a class
> is different.
A "closed class" directive however is an explicit specification that
makes the intention of the designer explicit in the program. Since
it would solve the puzzle in a rather elegant and explicit way, I
thought that it was interesting to mention.
All the best,
> Daan Leijen wrote:
>> Lennart Augustsson wrote:
>>> So in what sense is this really ambiguous?
>>> I think it would be quite reasonable to allow the Puzzle module
>>> to compile, resolving `a' to be Bool. I.e., if there is only one
>>> instance that can satisfy a constraint and there is no possibility
>>> of adding instances outside the compiled module, I think resolving the
>>> overloading makes sense.
>> You may be interested in a recent paper by Bastiaan Heeren and Juriaan
>> Hage  about type class directives. The "closed" directive in that
>> paper is more or less implied by your example. Here is a quote from
>> the paper:
>> "The main advantage of a closed type class is that we know the fixed
>> set of instances. Using this knowledge, we can influence the type
>> inference process. As discussed in the introduction to Section 2, we
>> can reject definitions early on (in case the set of instances for a
>> certain type class is empty) or improve a type variable to a certain
>> type (in case the set of instances is a singleton)."
>> All the best,
>> -- Daan Leijen.
>>  http://www.cs.uu.nl/~bastiaan/papers.html#typeclassdirectives
>> (to appear in PADL 2005)
>>> -- Lennart
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